Such a coordinate measuring machine and a corresponding method for operating such a coordinate measuring machine are known from U.S. Pat. No. 7,599,813. Inter alia, a coordinate measuring machine of the type mentioned in the introduction is disclosed therein. In the case of the coordinate measuring machine shown in association with the embodiments, this is a coordinate measuring machine of the so called gantry type. The latter includes a measurement table as workpiece support for mounting a workpiece to be measured. The sensor used to scan the surface of the workpiece is moved relative to the workpiece to be measured in the three mutually perpendicular coordinate directions (x, z, y) via a gantry mechanism. For this purpose, the mechanism includes a first measurement slide in the form of a gantry, which spans the measurement table and which is guided movably in a first coordinate direction on two parallel guides arranged laterally with respect to the measurement table. The first measurement slide (gantry) is driven via a first drive, which drives the first measurement slide along a first guide of the guides, and is driven via a second drive, which drives the first measurement slide along the second guide of the two guides. Along the cross beam of the first measurement slide, the cross beam spanning the measurement table horizontally, a second measurement slide is guided movably in a second coordinate direction, wherein the second measurement slide is assigned a position measuring system in the form of a scale and an associated scale sensor, via which the position of the second measurement slide relative to the first measurement slide can be determined. Moreover, the coordinate measuring machine includes a controller, by which the first drive and the second drive can be actuated.
In one particularly advantageous embodiment, the first and second drives of the first measurement slide (that is, of the gantry) are actuated by means of a multistage regulating circuit. In this case, the regulating circuit of both drives includes a jointly used position regulator, the output of which is applied to a regulator designated as a synchronous regulator. The synchronous regulator includes two mutually separate speed regulators, wherein the output of the first speed regulator is applied to the input of the first drive regulating circuit and the output of the second speed regulator is applied to the input of the second drive regulating circuit.
The position regulator is a standard position regulator for a movement axis, wherein the position regulator regulates the position of the gantry not along the movement axis of only one drive, but rather along a fictitious drive axis lying in the center between the two movement axes of the two drives for the first measurement slide. For this purpose, the mechanism for measuring the position of the first measurement slide includes two position measuring systems, wherein position measurement values of the first measurement slide relative to the first guide are ascertained via a first of the position measuring systems and position measurement values of the first measurement slide relative to the second guide are ascertained via the second position measuring system. From these aforesaid first position measurement values of the first measurement slide and the second position measurement values of the first measurement slide, an average value is then calculated and fed back as feedback to the position regulator. A reference variable for the position regulator is a setpoint position.
The output of the position regulator is applied as a reference variable to both speed regulators of the synchronous regulator.
In a first variant, the time derivative of the average value is used as a feedback variable of the speed regulators.
In a second variant, a difference speed resulting from the difference value of the measured speed of the first drive and of the second drive is used as a feedback variable of the speed regulators. In this case, firstly the difference speed itself, and also the single time derivative of the difference speed and the double time derivative of the difference speed are fed back to the two speed regulators, wherein the feedback is done with different signs; that is, that at one speed regulator the feedback variable is subtracted from the reference variable and at the other speed regulator the feedback variable is added to the reference variable.
The two speed regulators then yield, as output, speed values for the drive regulating circuit of the first drive and for the drive regulating circuit of the second drive for driving the first measurement slide.
The special feature of the coordinate measuring machine known from the relevant document can be seen in the use of two separate drives via which the first measurement slide is driven in the first coordinate measuring direction. As a result, it is possible purely in principle to compensate for torques which would occur in the case of a drive of the first measurement slide with only one drive on only one side of the first measurement slide. However, the compensation of the torques is possible only very unsatisfactorily with the controller disclosed. This has various causes. Firstly, in the case of a movement of the second measurement slide in the second coordinate direction, the center of gravity of the entire mechanism shifts in the first coordinate direction. This leads to altered mass moments of inertia of the mechanism which are not taken into account in the regulator of the controller in accordance with the documents cited above. Furthermore, the abovementioned regulator can react to resultant torques, resulting from the forces of the first and second drives, only if, on account of the resultant torque, the first measurement slide is already rotating about the rotation axis perpendicular to the first and second coordinate directions.